1. Field of the Invention
The present invention relates generally to the field of optical lens systems, and specifically, to athermalized mounts for lenses to compensate for thermal image defects due to temperature changes.
2. Background Information
Typically, lenses are used in many applications such as in cameras and camcorders. However, lenses have a potentially serious limitation in those applications involving moderate changes in temperature. In particular, changes in temperature produce a shift of a lens image plane and can also modify the aberration balance of the lens from the nominal lens design at room temperature (referred to as "thermal image defects"). The ability to effectively compensate for thermal image defects (referred to as "athermalization") opens the use of lenses to a large class of applications that do not permit active refocusing with variations in temperature.
The principal cause of these thermal image defects is the change in the refractive index of the lens material. The change in refractive index is called the dN/dT of the material and is typically -0.000120 per degree Centigrade for plastic lenses. For example, as the temperature increases, the refractive index for a plastic lens decreases which increases the focal length (since light is bent less) and the image plane of the plastic lens. Optical glasses, on the other hand, have dN/dT values in the +0.000003 per degree Centigrade range.
A secondary contributory factor for thermal image defects in lenses is due to the Coefficient of Thermal Expansion ("CTE"). As temperature increases, the radius of curvature of a lens becomes longer and its thickness increases. This causes the focal length to increase and thereby shift the image plane location further away from the lens. The CTE is much larger for plastic than glass. Consequently, thermal image defects is more severe in plastic lenses.
For optical systems that operate in the moderate to extreme temperature range, several solutions exist to athermalize for the degradation of image quality due to the change in the image plane location with temperature. Active athermalization involves using a motor to drive the lens or image plane as a function of temperature. As is apparent, this solution is undesirable because it requires power and adds to the cost of the lens system. Passive athermalization involves using a design that automatically corrects for the shift in the image plane with temperature.
One possible passive athermalization mechanism involves the combination of plastic materials and refractive power distributions in the lenses in a way that minimizes the thermal shift of focus and the change in aberration balance. This mechanism typically uses a hybrid glass/plastic lens design where most of the positive refractive power is vested in a glass element and the weakly-powered plastic elements are used for aberration balance. However, even for these hybrid glass/plastic lens designs, the thermal focal shift can be a serious concern and active focusing of the optical system is often the only real practical solution. Moreover, the use of a hybrid lens design requires a glass lens which increases cost.
FIG. 1 illustrates a simple singlet plastic lens element 10. Referring to FIG. 1, the singlet plastic lens element 10 is made from acrylic ("PMMA"), has a f/2.0 relative aperture, and an effective focal length ("EFL"), L, to an image plane 12, of 25.4 millimeters ("mm"). When the temperature is increased from +20.degree. C. to +30.degree. C., the distance of the image plane 12 from the rear vertex increases by .DELTA.L or +0.071 mm.
FIG. 2 illustrates a prior art athermalizing plastic lens element apparatus. Referring to FIG. 2, the plastic lens element 10 is the same as shown in FIG. 1 and is supported by a mechanical support element 14 which is made from a high expansive material (e.g., acrylic). As the temperature increases and the image plane shifts away from the lens element, the mechanical support element 14 expands axially in the -X direction (i.e., opposite to the shift of the image plane) to partially compensate for the movement of the image plane 12. However, the compensation is almost always far from matching the image plane movement. For example, for a temperature increase from +20.degree. C. to +30.degree. C., the image plane increases by .DELTA.L or +0.071 mm and the mechanical support element expands axially in the -X direction by .DELTA.C or +0.019. The net result is an under compensation of 0.052 mm.
FIG. 3 illustrates another form of the prior art athermalized plastic lens element apparatus. In this embodiment, three nested cylindrical support elements are used to support the plastic lens element. Referring to FIG. 3, an inner cylinder 14 is made from a high expansive material (e.g., acrylic), a middle cylinder 16 is made from a low expansive material (e.g., Invar), and an outer cylinder 18 is made from a high expansive plastic. As temperature increases, the inner cylinder 14 expands axially in the -X direction as in FIG. 2. The middle cylinder 16 has an almost zero CTE and does not expand with temperature. Similar to the inner cylinder 14, the outer cylinder 18 expands axially in the -X direction to move the middle cylinder 16 in the -X direction which in turn further moves the inner cylinder 14 in the -X direction. In this embodiment, the inner and outer cylinders 14 and 18 both expand in an additive manner to effectively double the length of the axial expansion in the -X direction and provide greater compensation.
However, this embodiment usually cannot match the full image plane movement. For example, for a temperature increases from +20.degree. C. to +30.degree. C., this embodiment provides an axial compensation of .DELTA.C or +0.037 mm. The net result is still an under compensation of +0.034 mm. Moreover, as the inner cylinder 14 expands with temperature, it tends to lock up hard against the surface of the middle cylinder 16. As the temperature decreases, the outer cylinder 18 contracts and tends to lock up against the surface of the middle cylinder 16.
These comparisons were performed with only a +10.degree. C. change in temperature. The results are even more dramatic with higher temperature changes. Thus, prior art solutions do not provide a sufficient compensation for thermal image defects. The problem is complicated by the fact that multiple lens systems typically tend to be very compact and there is not enough room between the lenses to provide a long enough mechanical support member to compensate for the thermal image defects.
Accordingly, there is a need in the art for a method and apparatus to effectively compensate for thermal image defects in lenses due to changes in temperature while maintaining a compact design.